EXPERIMENTAL RESULTS ON INSTABILITY CAUSED BY NONUNIFORM CURRENT DISTRIBUTION IN THE 30 KA NBTI DEMO POLOIDAL COIL (DPC-U) CONDUCTOR

Two 30 kA, NbTi Demo Poloidal Coils, DPC-U1 and DPC-U2, were fabricate d and tested in the Demo Poloidal Coil project at the Japan Atomic Ene rgy Research Institute. DPC-UI and -U2 have a large current, forced fl ow cooling, cable-in-conduit conductor, which is composed of 486 stran ds. The strand surfaces are insulated by formvar to reduce coupling lo sses between the strands. DPC-U1 and -U2 reached their design current, but exhibited instability during charge, in many cases resulting in a coil quench. Such a quench occurred even at a current one-tenth of th e conductor critical current. To clarify the cause of the instability, a detailed investigation on the quench current and normal voltage beh aviour was carried out by charging the coil in several ways to the coi l quench, and by measuring the stability of the coil at a current of 1 6-21.5 kA. These experimental results revealed the existence of non-un iformity of current distribution among the strands in the conductor, e ven under slow charging. This non-uniformity of current distribution c aused the instability of the coil. The time constant of current redist ribution is very large due to the insulation between the strands. Howe ver, if part of the conductor can be forced to go normal without coil quench occurring, a redistribution of current takes place and the curr ent distribution becomes more uniform. it was then demonstrated that t he current distribution could become uniform by applying heat to the c onductor to generate intentional normalcy. Consequently, the possibili ty of stable operation of the DPC-U was suggested.